This invention relates to static power converter arrangements and, more particularly, to a static power converter arrangement to provide unlimited voltage.
Conventional static power converter arrangements are limited by the shortcomings of available power semiconductor devices. Direct parallel and/or series operation of power semiconductor devices requires excessive device derating and careful matching of the devices to ensure voltage and current sharing and thermal stability.
The Paice et al. U.S. Pat. No. 4,674,024 discloses a medium voltage modular inverter and control system for a variable frequency inverter motor drive in which low voltage inverter modules are combined to form a medium voltage multi-inverter system by connecting standard modules having an H-bridge of semiconductor switches supplied by isolated individual DC power sources in a string and connecting strings of such inverter modules in a wye formation to provide pulse width modulated power to control the variable speed motor without requiring an output transformer.
The Hammond U.S. Pat. No. 5,625,545 discloses an electric drive apparatus using input transformers with multiple, phase-shifted, isolated secondaries, each feeding an isolated power module, the modules being connected in series to achieve a high output motor voltage. Control of the power modules is achieved via normal and complementary triangle waves compared with an externally generated reference wave. Triangle waves for serially connected modules are delayed in time from other triangle waves in that module string.
The Weiss U.S. Pat. No. 4,349,772 discloses a DC inverter power supply using plural inverter circuits in which less than all of the inverter circuits are included in the power supply when the motor torque requirement is reduced.
The Stacey U.S. Pat. No. 5,515,264 discloses a method for harmonic neutralization of an electric drive by a plurality of 6-pulse bridges coupled to a common DC source. Quasi-harmonic neutralization inverter systems providing up to 120 pulses, using zigzag interphase transformers at the outputs of each 6-pulse bridge, are disclosed. A successive staggering technique for harmonic neutralization using interphase transformers is disclosed in the Stacey et al. U.S. Pat. No. 5,337,227. In addition, the Bausch U.S. Pat. No. 5,426,354 discloses an H-bridge configuration of power transistors which generate pulse width modulated voltage waveforms which are in turn supplied to various phase terminals of the stator of a motor.
In each of the prior art arrangements, however, the semiconductor power transistors are operated in a two-dimensional arrangement in accordance with a one-dimensional pulse width modulated control algorithm. Moreover, in those arrangements, the semiconductor power transistors are operated at less than maximum efficiency, especially when reduced power is required from the system.